XMM-Newton study of SNR W28 : the thermal & non-thermal emission Ping Zhou University of Manitoba, Nanjing University Collaborators: Samar Safi-Harb University of Manitoba (supervisor) Yang Chen Nanjing University (supervisor) Xiao Zhang Nanjing University The Cosmic Kaleidoscope 16 Aug, 2012 We acknowledge the funding support of CSC (China Scholarship Council) and NSERC (Natural Sciences and Engineering Research Council of Canada).

OUTLINE Introduction previous observations of W28 XMM study the SNR interior Spatially resolved spectroscopic analysis interpretation of the thermal emission the northeastern shell Spectroscopic analysis interpretation of the non-thermal emission

Introduction: a special SNR ROSAT X-ray ( keV; Rho & Borkowski 2004) Contours: VLA1.4GHz (Dubner et al 2000) Mixed-Morphology SNR (MMSNR) with a bright X-ray shell shell CO (2-1) image (Nicholas et al. 2010) Black contours: H.E.S.S TeV γ-ray significance Interacting with molecular clouds, hadronic-origin-Cosmic- Ray (Aharonian et al. 2008) shell

XMM observations 4 archival XMM observations with 3 different pointings. Red: KeV Blue: keV Contour: VLA 1.4GHz ExposurePN (ks)MOS (ks) Northeast2430 Center54 North11 solid red region: source dashed red region: background A spatially resolved spectroscopic analysis Adequately select nearby background

Central gas: spectral results a large gradient of NH, temperature and density hotdiffusecolddense vnei+ vmekal \\\] \ from east to west: N H generally kT_c generally kT_h generally abundance <1 VNEI (cold)+VMEKAL(hot) RCCNC3C2C1C0 Δχ^2 (d.o.f)1.32 (619)1.26 (453)1.05 (475)0.98 (437)1.18 (495)1.12 (391) NH (1E22 cm^-2)0,550,720,590,390,430,36 kT_c (keV)0.36 ( ) 0.28 ( ) 0.31 ( ) 0.57 ( ) 0.54 ( ) 0.64 ( ) COLD Tau_c (10E11 s cm^-3) 4.34 ( ) 5.14 ( ) 4.91 (>1.10) ( ) 7.28 (>1.48) n_c (cm^-3)1,481,792,010,731,170,91 HOT kT_h (keV)0.77 ( ) 0.78 ( ) 0.77 ( ) 1.20 ( ) 1.00 ( ) 1.40 ( ) n_h (cm^-3)0,690,640,810,350,630,42

12CO 3-2 (JCMT; Arikawa et al. 1999) Hα emission (SuperCOSMOS Hα Survey) X-ray keV (XMM) Contour: VLA 1.4GHz Interacting with dense material (CO and Hα filaments) in the east and inner radio ridge. Central gas: interpretation environment explains the gradient of foreground absorption density temperature filamentary Hα emission diffuse Hα emission

Popular scenarios: 1. thermal conduction (Shelton et al 1999; Cox et al. 1999) 2. cloudlet evaporation (White & Long 1991) thermal conduction -- Not efficient a large gradient of temperature and density from east to west. the low mass of X-ray-emitting gas (~30Msun) and the X-ray clumpiness are NOT consistent with the thermal conduction model (Rho & Borkowski 2002)colddense hotdiffuse thermal conduction Central gas: interpretation

Advantages: clumpy X-ray structures. the colder component is denser and has a lower ionization timescale (model predicts the most recently evaporated material should be more under-ionized embeded in a hotter matrix) bright, diffuse Hα emission and low ratios of [SII]/Hα Cloudlet evaporation should be a dispensable and important process. An improved model is required. cloudlet evaporation Central gas: interpretation In addition to nonuniform environment, Cloudlet evaporation is an indispensable and important process to influence the gas properties. Improved model is also required. Disadvantages: X-ray surface brightness is too sharply peaked at the center (Rho & Borkowski 2002). the gradient of temperature and density from west to east (rather than a symmetrical distribution). cloudlet evaporation

VNEI+power law model soft component(``shell”) : kT~0.3 keV abundance<1 non-thermal X-ray on the northeastern shell Γ~1.5 ( ) Γ~1.9 ( ) Γ~1.1 ( ) Flat photon index (appears flatter in the south region ``S2”) No evidence of Fe Kα line at 6.4keV shell S1 S2

Possible origin of the non- thermal emission 2. Inverse Compton scattering? not important in soft X-ray band, unless B > U CMB requires electrons accelerated to energy > tens of TeV. hard for old SNR as W28 (shock velocity~80km/s) X-ray spectrum is loss-steepened which results a typical photon index value > observed value Γ~ Synchrotron? The hard non-thermal continuum ( keV, in region ``Shell”) :

Possible origin of the non- thermal emission a favorable explanation for the hard spectra in the MC- SNR interaction region. also need further calculation for the luminosity. 3. Non-thermal bremsstrahlung Bremsstrahlung radiation in X-ray can be produced by electrons with energy < tens of MeV. Coulomb loss dominates and flattens the spectrum. for Γ= , α= successfully explain the hard spectrum emitting clumps in γ Cygni (Uchiyama et al. 2002; Bykov et al. 2004).

Summary W28 is an MMSNR with a deformed X-ray shell and diffuse blobby X-ray emission in the interior. vnei (cold)+vmekal (hot) model well reproduces the spectra in the SNR interior, with a spatially varying spectral properties. thermal conduction is not efficient. the cloudlet evaporation model, together with a nonuniform environment, can explain many of the plasma properties. The shell gas is best characterized by a thermal plus a non-thermal model, with a hard photon index non-thermal bremsstrahlung is the most feasible origin.

Thank you!

4. non-thermal X-rays from Secondary leptons? Possible origin of the non-thermal emission The hadronic origin of γ-ray is widely accepted The model of Gabici et al. (2009) is adopted Red: synchrotron green: bremsstrahlung black: pion decay bow tie: observed X-ray

ASCA, NEI model ( Rho & Borkowski 2002) An elevated hard tail compared to the model Energy (keV) XMM PN 18ks obs, APEC+power law model ( Ueno et al. 2003) A flat photon index Γ~1.3 ( ) is found. However, no discussion about the origin of the non-thermal X-ray Introduction: non-thermal X-ray on the northeastern shell?

two XMM observations in NE PN and MOS data are used to joint fit the spectra VNEI+power law model compare the powerlaw index between the northern region ``S1”and southern region ``S2”. Energy cuts below 7keV Compare to XMM study of Ueno et al (2003) XMM hard X-ray ( keV)

Intro: debate on the central gas Popular scenarios for central hot gas: 1. cloudlet evaporation 2. thermal conduction others: reflected shock, projection effects ROSAT and ASCA studies: (Rho & Borkowski) W28 poses a challenge for existing models. X-ray emission at its center is a “fossil” radiation ASCA study (Kawasaki et al) Search for evidence of overionization, but none is found SUZAKU (Sawada & Koyama) claim central plasma is overionized Recombining plasma? Einstein (Long et al. 1991) Cloudlet evaporation Young age of W28 (~2500yr)

γ-ray in W28 (Nicholas et al. 2010) (Aharonian et al. 2010) Fermi LAT 2-10 GeV count map Black contour: VLA 90cm Abdo et al GeV TeV

Chandra and optical observation by Keohane et al upper:ROSAT X-ray image bottom:The ratio of [SII]/Hα In W28 center: a)X-ray emitting gas is clumpy; b)Low ratio of[SII]/Hα

Spectral extraction region for ASCA, SUZAKU and XMM Black rectangle: ASCA Black circle: SUZAKU

Spectra for small-scale regions in W28 interior